Two of the four known groups of the human HIV virus that causes AIDS originated in western lowland gorillas in Africa, an international research team has determined.
The main type of human immunodeficiency virus, HIV-1, is made up of four groups, each the result of a separate cross-species leap of the virus from apes to humans, they say.
Two of those lineages originated in gorillas in Cameroon before infecting people, probably through the consumption of hunted bush meat, they explain.
Scientists say they now understand the virus did not make one single leap from animals to human, but rather there were several such jumps.
Previous research has identified one HIV-1 strain, group M, as coming from chimpanzees in southern Camaroon. That strain eventually infected some 40 million people worldwide and led to the AIDS pandemic.
Another strain, group N, is geographically limited and has been found in very few humans.
New research has linked the remaining two strains, groups O and P, to the gorillas, also found in southern Cameroon.
"Thus, both chimpanzees and gorillas harbor viruses that are capable of crossing the species barrier to humans and causing major disease outbreaks," says virologist Martine Peeters of the University of Montpellier in France.
Group O probably emerged at the beginning of the 20th century, the researchers say, while group P arose sometime later in the century.
All four groups of human HIV-1 virus almost certainly began as similar viruses in monkeys and apes, known as simian immunodeficiency viruses, or S.I.V., they say.
Peeters and her colleagues studies a number of primate species, and in 2006 discovered the first S.I.V. known to infect gorillas.
Reconstructing the history of gorilla S.I.V., they determined the gorillas originally acquired the virus from chimpanzees at some point in the past, and then on two occasions humans acquired the gorilla S.I.V., creating the two HIV-1 groups O and P.
Unlike the chimpanzee-derived human virus that caused the worldwide AIDS pandemic, the O and P groups have for the most part remained in Cameroon.
Since both the chimpanzee and gorilla viruses have adapted themselves to human hosts, it's mostly a matter of luck that the gorilla-derived version has remained in the small African country, the researchers say.
In the mid-1900s, someone infected with H.I.V.-1 Group M, evolved from the chimp S.I.V., traveled from Cameroon to the rapidly growing city of Leopoldville (now Kinshasa) in the Democratic Republic of Congo, where it thrived under what for it were perfect conditions.
"M got into Kinshasa and went boom," says University of Pennsylvania microbiologist Beatrice H. Hahn, a co-author of the new study. "If the O group had similar chances, we might have had a second epidemic."
